Mash seam welding is known as a welding method which places two metal plates to overlap each other at their end portions and presses the overlapping portions with a pair of electrode wheels while flowing welding currents to continuously weld the overlapping portions, and simultaneously to cause a joint portion heated at a high temperature and softened to be rolled with the electrode wheels, thereby reducing the thickness of the joint portion.
Conventional techniques of a mash seam welding apparatus for joining metal plates with large thickness are described in Patent Documents 1 and 2, for example.
As a problem with joining of thick metal plates, Patent Document 1 raises a problem that the metal plates (materials to be welded) need to be significantly bent as illustrated in FIG. 13 of Patent Document 1 and a large force is applied to clamp devices, so that the clamp devices are deformed or broken. As means for solving the problem, Patent Document 1 describes that first and second clamp position adjustment means are provided to adjust distances between a joint portion of the metal plates and clamp positions of the first and second clamp devices. When the thicknesses of the metal plates are large, the distances between the joint portion and the clamp devices are set to be large so as to reduce curvatures of the metal plates and reduce force acting on the clamp devices, thereby preventing the clamp devices from being broken.
As a problem with joining of high-strength steel strips, Patent Document 2 raises the problems with reference to FIG. 8 thereof that since pressing forces applied to the electrode wheels need to be high and a corner of the upper metal plate (indicated by reference numeral 20 in FIG. 8 of Patent Document 2) and the electrode wheel contact with each other, the electrode wheel is significantly damaged, and that since a welding current is concentrated in a corner of the lower metal plate (indicated by reference numeral 21 in FIG. 8 of Patent Document 2), welding surface flash (phenomenon in which the corner is melt and dispersed due to the excessive current concentration therein and the spatters remain on the metal plate) is generated, and a desired welding result cannot be obtained. When thick metal plates are to be joined, since the volume of a joint portion becomes large, a current needs to be high and since the rigidity of the metal plates is high, welding force to be applied by the electrode wheels need to be increased, and therefore, the same problem arises from the same principle as the joining of the high-strength steel strips. To address such problems, Patent Document 2 describes that first pressing rolls are provided to press the overlapping portions of the metal plates, and after the overlapping portions of the metal plates are pressed by the first pressing rolls, the electrode wheels are brought into contact with the pressed overlapping portions and applied with electric currents to weld the overlapping portions, thereby suppressing occurrence of scratches on the electrode wheel surfaces and generation of welding surface flash.